Permanent magnet twister

ABSTRACT

A permanent magnet rectangular bar shaped to form a helix having a centralore and a longitudinal axis. The magnetic orientation of the helical permanent magnet bar is along the longitudinal axis forming a transverse helical magnetic field. An electron beam or charged particle passing near the longitudinal axis experiences acceleration causing the charged particles to radiate creating a high energy radiation source. In another embodiment, first and second permanent magnet helical bars are intertwined together, forming a cylindrical tube, with each of the intertwined helical bars having a magnetic orientation along the longitudinal axis in opposing directions. In another embodiment of the present invention, an iron ribbon or other high permeability magnetic material is interposed between cut-out portions between first and second intertwined helical permanent magnet bars. This effectively augments the transverse magnetic field. A relatively compact permanent magnetic structure that is relatively simple having a desired transverse helical magnetic field is made possible for such applications as free electron lasers or millimeter/microwave radar devices.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby and for the government of the United States of America forgovernmental purposes without the payment to me of any royalty hereon.

RELATED APPLICATION

"A Chrion Twister" which has been partially assigned to the sameassignee, has been filed in the United States Patent and TrademarkOffice and is related to this application.

FIELD OF THE INVENTION

The present invention relates in general to a permanent magnet structureused as a twister and more particularly to a permanent magnet structure,creating a magnetic field that causes an electron to have a helicaltrajectory.

BACKGROUND OF THE INVENTION

In many devices such as radar, free electron lasers, and other highenergy radiation sources such as high powered millimeter or microwavedevices, there is a need for providing a helically oriented transversemagnetic field. Such permanent magnetic devices are known, for example,the permanent magnetic structure disclosed in U.S. Pat. No. 4,764,743entitled "Permanent Magnet Structures For The Production Of TransverseHelical Fields" issuing to Leupold et al on Aug. 16, 1988, which isherein incorporated by reference. Therein disclosed is a permanentmagnet structure formed from a multiplicity of permanent magnetsegments, with the segments displaced radially from each otherprogressively along the longitudinal axis. Another permanent magnetstructure is disclosed in U.S. Pat. No. 4,994,778 entitled "AdjustableTwister" issuing to Leupold on Feb. 19, 1991, which is hereinincorporated by reference. Therein disclosed is a permanent magnetstructure comprising a linear array of hollow cylindrical flux sourcestructures nested one within another about a common central axis andfree to rotate about the axis providing a periodic magnetic structuresuitable for use as a helical wiggler or a twister. Another permanentmagnet structure is disclosed in U.S. Pat. No. 5,099,217 entitled"Constant Gap Cladded Twister" and issuing to Leupold on Mar. 24, 1992,which is herein incorporated by reference. Therein disclosed is acladded magnet twister with each segment being displaced radially inequal angular segments along the central axis having a plurality ofcladding magnets and opposing pole pieces having convex faces which aredirected inward toward one another. While these prior permanent magnetstructures produce a desired magnetic field within their central workingspace, they all have a relatively complex permanent magnet structurewith a multitude of different magnets. Therefore, there is a need for asimpler permanent magnet structure having less volume and weight thatproduces a desirable transverse helical magnetic field.

SUMMARY OF THE INVENTION

The present invention, in one embodiment, comprises a permanent magnetbar shaped to form a helix and having a central bore or working spacewith a longitudinal axis. The permanent magnet helical bar is magnetizedwith a magnetic orientation substantially parallel to the longitudinalaxis. Additionally, the longitudinal or axial thickness of the bar issubstantially equal to the longitudinal or axial spacing distancebetween the helical coils of the permanent magnet bar. A desirabletransverse helical magnetic field is thereby formed within the bore orworking space of the permanent magnet structure. In another embodimentof the present invention, a second permanent magnet helical bar isintertwined with a first permanent magnet helical bar such that thelongitudinal magnetic orientation of each of the helical bars isopposite to each other, thereby increasing the transverse magnetic fieldand resulting power output of the device. In a third embodiment of thepresent invention, a high permeability magnetic material, such as iron,is intertwined between a first intertwined helical bar and a secondintertwined helical bar, thereby augmenting the transverse magneticfield on axis.

Accordingly, it is an object of the present invention to form adesirable transverse helical magnetic field in a central working spaceor bore.

It is an advantage of the present invention that it is a relativelysimple structure that is easy to manufacture and of a compact size andweight.

It is a feature of the present invention that one or more permanentmagnet helical bars are used.

It is another feature of the present invention that a helical iron barcan be placed between adjacent coils of the helical permanent magnet.

These and other objects, advantages, and features will become readilyapparent in view of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of the present invention.

FIG. 1B is a top plan view of the embodiment illustrated in FIG. 1A.

FIG. 1C is a cross section taken along line 1C--1C in FIG. 1B.

FIG. 2A is a perspective view illustrating a second embodiment of thepresent invention.

FIG. 2B is a cross section view of the second embodiment illustrated inFIG. 2A.

FIG. 3 schematically illustrates a third embodiment of the presentinvention.

FIG. 4 schematically illustrates a fourth embodiment of the presentinvention.

FIG. 5 is a perspective view of an embodiment similar to the embodimentof FIG. 1 illustrating the resultant magnetic field.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A, 1B, and 1C illustrate a first embodiment of the simplest formof the present invention. FIG. 1A illustrates a helical bar that has asubstantially rectangular cross section. The helical bar 10 may bemanufactured by any conventional means. For example, a high cohesivepermanent magnet in the form of a cylinder or pipe that is magnetized inthe longitudinal or axial direction could be cut so as to form thehelical bar 10. Accordingly, the helical bar 10 may be made from anyhigh cohesive permanent magnet material of high coercivity, i.e. higherthan its remanance measured in Gaussian units. The helical bar 10 has atop surface 12 and a bottom surface 14. The helical bar 10 is magnetizedin the axial direction illustrated by arrows 16, such that magneticnorth is on the top surface 12 and magnetic south is on the bottomsurface 14. The helical bar 10 is formed around a central axis 18 alongwhich an electron beam may be projected from electron beam source 22.The helical bar 10 has a radial width W, and an axial thickness T₁between the top surface 12 and the bottom surface 14. The spacingbetween adjacent coils of the helical bar is indicated as T₂. Thepreferred structure is such that the axial thickness T₁ is equal to thecoil spacing T₂. As in FIG's 1 and 5 there will only be a radialcomponent of the field on axis where the beam action takes place.Accordingly, the axial thickness T₁ is that distance along the helicalaxis in one-half turn of a coil. In this way, the north pole chargedistribution is always diametrically opposed to the south poledistribution so that the transverse field at the central axis 18 ismaximized. In that way there will be only be a radial component of thefield on axis where the beam action takes place. Off-axis there will bean axial component as well which helps focus the beam to keep it on ornear the axis 18. It should be noted that the axial spacing between thenorth and south pole surfaces should be substantially greater than thebore diameter. Otherwise, much, or in extreme cases most of the fluxwill pass between adjacent polar surfaces rather than transverselyacross the bore. The magnetic field has axial components at points thatare not on axis 18. The transverse component of the magnetic fieldimparts a circular motion to the electron beam or charged particles,which, when added vectorially to the axial translational motion, resultsin a helical trajectory. The motion having a rotary component, thereforeinvolves acceleration which causes the charged particle to radiate. Ifthe magnetic field, B, is such that its product with the period (T₁ +T₂)is approximately equal to unity, or 1, when B is measured in Telsa's and(T₁ +T₂) is measured in centimeters, laser action may form coherentradiation from all parts of the beam forming a power source ofcircularly polarized radiation. The magnetic fields produced accordingto the structure of this first embodiment compare favorably to those ofother, much more complicated structures. The magnitude of the magneticfield in the working space or bore may be increased by increasing theradial width W. However, this will add mass or weight to the structureand quickly becomes ineffective if the width becomes much greater thanthe distance between successive windings. The present invention canproduce a relatively large field, in the order of 1800 Oe, in a smallerstructure with less assembled parts than is conventionally possible.FIG. 1B is a top plan view illustrating the helical bar 10 and the bore20 formed thereby. FIG. 1C is a cross section taken along line 1C--1C inFIG. 1B, more clearly illustrating the rectangular structure of thehelical bar 10. FIG. 1C also more clearly illustrates that the northpole charge distribution is preferably diametrically opposed to thesouth pole distribution so that the transverse field at the central axis18 is maximized and in a diametric direction. Arrows 17 illustrate thegeneral direction of the magnetic field created within the bore formedby the helical bar 10. It should be appreciated that for convenience ofillustration the bore formed by the helical bar 10 has a relativelylarge diameter d with respect to the adjacent coil spacing T₂. However,in an actual or preferred device the diameter of the bore d will besubstantially less than the adjacent coil spacing T₂. Accordingly, adesired relatively strong transverse magnetic field, illustrated byarrows 17, will be created and dominate rather than a longitudinalmagnetic field between adjacent coils.

FIGS. 2A and 2B illustrate a second embodiment of the present inventionhaving a first helical permanent magnet bar 110 and a second helicalpermanent magnet bar 111 intertwined to form a cylindrical tube or pipe.The cylindrical tube or pipe is preferably a right cylinder. The firsthelical bar 110 has a magnetic orientation represented by arrows 116that is opposite or opposed to the magnetic orientation represented byarrows 115 of the second helical bar 111. Preferably, the longitudinalor axial thickness of the first helical bar 110 is equal to thelongitudinal or axial thickness of the second helical bar 111. A centralworking space or bore is formed within the intertwined first and secondhelical bar permanent magnets 110 and 111. The bore has a longitudinalaxis 118. FIG. 2B is an axial cross section of FIG. 2A, which moreclearly illustrates the magnetic orientation of adjacent coil portionsof the first and second helical bars 110 and 111. The use of a firstpermanent magnet helical bar 110 and a second permanent magnet helicalbar 111, having opposing magnetic orientations along a longitudinalaxis, effectively doubles the pole density at each layer betweenadjacent coils, thereby doubling the transverse magnetic field.Accordingly, an electron beam traveling along or parallel to thelongitudinal axis 118 is subject to twice the acceleration compared tothe use of only one helical bar of the first embodiment of the presentinvention, illustrated in FIGS. 1A-1C. Since the power of a freeelectron laser varies as the square of the accelerating field thatcauses the electron to follow a helical path, the power output of such adevice would be quadruple that of a single helical bar free electronlaser or twister formed according to the first embodiment, illustratedin FIGS. 1A-1C. The second embodiment provides a stronger transversemagnetic field in a compact space.

FIGS. 3 and 4 illustrate embodiments that use iron or other highpermeability magnetic material placed between the adjacent first andsecond helical bars 111 and 110, illustrated in FIGS. 2A-2B. Thisresults in the transverse magnetic field on axis to be augmented becauseof the easy magnetic flux path afforded through the iron or other highpermeability magnetic material. In FIG. 3, a first permanent magnethelical bar 210 having a magnetic orientation, illustrated by arrows216, parallel to the longitudinal axis 218, is intertwined with a secondpermanent magnet helical bar 211, having a magnetic orientationillustrated by arrows 215, in a direction opposite or opposed to that ofthe first helical permanent magnet bar 210. Helical bars of iron orother high permeability magnetic material are intertwined between carvedout portions of the adjacent first and second helical intertwined bars210 and 211. A helical iron bar is placed between each surface of thefirst and second helical intertwined bars 210 and 211. Therefore, thereare two helical iron bars 221 and 222 placed between the boundary of thefirst and second helical bars 210 and 211, with iron bar 221 forming anorth pole and iron bar 222 forming a south pole. However, the iron baralso has the detrimental effect of allowing additional magnetic fluxleakage to the outside of the permanent magnet structure. To reduce themagnetic flux leakage, the iron bars 221 and 222 can be shortened orindented on the outside so that flux passage across the interior workingspace or bore is favored. Therefore, the iron bars 221 and 222 extend aradial distance outward less than that of the exterior surface of theintertwined permanent magnet bars 210 and 211.

FIG. 4 illustrates another embodiment having intertwined permanentmagnet helical bars 310 and 311 with respective opposing magneticorientations illustrated by arrows 316 and 315. The permanent magnethelical bars 310 and 311 have a trapezoidal cross section. A centralworking space or bore having a longitudinal axis 318 is thereby formed.Intertwined between the first helical permanent magnet bar 310 and thesecond helical permanent magnet bar 311 are first and second ironhelical ribbons 321 and 322. The helical 321 and 322 have a triangularcross section. The triangular cross section of the intertwined helicaliron 321 and 322 has a base adjacent the interior surface and a vertexadjacent the exterior surface. This facilitates magnetic flux flow inthe interior and restricts magnetic flux flow on the outside or externalsurface of the permanent magnet structure. The use of helical iron barsillustrated in FIG. 3 or FIG. 4 results in a significant increase inmagnetic field or a sizable reduction in size for a constant magneticfield, both of which have great advantage in airborne or ballisticvehicles where lightweight and close packing are a necessity. It shouldbe appreciated that while the iron material 221, 222, 321, and 322 hasbeen described as an iron bar or ribbon, the iron or other highpermeability magnetic material could take other forms, such as beingformed by an initially fluid state that is permitted to flow between theadjacent coils.

FIG. 5 is a perspective view more clearly illustrating the magneticfield created by a helical magnet structure of FIG. 1. Permanent magnethelical bar 410 is illustrated in FIG. 5. The helical bar 410 has anorth pole surface N and a south pole surface S. As in FIG. 1, thehelical bar 410 has an axial thickness of T₁ and spacing betweenadjacent coils of T₂. A transverse magnetic field is created within thebore formed by the helical bar 410. The transverse magnetic fieldspirals or rotates as it progresses along the longitudinal axis of thehelical bar 410. Arrows 417 illustrate the transverse magnetic fieldrotating around the longitudinal axis of the helical bar 410. In thisfigure, FIG. 5, the heads of the arrows 417 point to the south polesurface S, and the tails of the arrows 417 point to the north polesurface N. The transverse rotating magnetic field is formed betweentransversely diametrically opposed north pole surfaces N and south polesurfaces S of the helical bar 410. Accordingly, the magnetic field willhave a radial component and an axial component perpendicular thereto.

Accordingly, the present invention, in using a permanent magnet helicalbar produces a working space having a relatively strong helicallyoriented transverse magnetic field or flux in a relatively compact andeasily manufactured shape. As a result, efficient and powerfulmillimeter and microwave sources can be made resulting in themanufacture of smaller and more powerful devices. Additionally, whileseveral embodiments have been illustrated and described, it will beobvious to those skilled in the art that various modifications may bemade without departing from the spirit and scope of this invention.

What is claimed is:
 1. A permanent magnet twister, comprising:apermanent magnet bar shaped in the form of a helix, said permanentmagnet bar having a central bore, a longitudinal axis, and a magneticorientation in a direction along said longitudinal axis; an electronbeam source, said electron beam source directing electrons near saidlongitudinal axis; said permanent magnet bar has a rectangular crosssection having an axial thickness along said longitudinal axis; saidpermanent magnet bar having a plurality of coils, each of said pluralityof coils spaced from each other a distance equal to the axial thicknessof said permanent magnet bar; wherein a transverse helical magneticfield is formed within the central bore causing an electron travelingnear said longitudinal axis to have acceleration causing said electronto radiate.
 2. A permanent magnet twister comprising:a first permanentmagnet bar, said first permanent magnet bar shaped in the form of ahelix having a first central bore and a first longitudinal axis, saidfirst permanent magnet bar having a first magnetic orientation in afirst direction along the first longitudinal axis; and a secondpermanent magnet bar, said second permanent magnet bar shaped in theform of a helix having a second central bore and a second longitudinalaxis, said second permanent magnet bar having a second magneticorientation in a second direction along the second longitudinal axis,said first permanent magnet bar and said second permanent magnet barbeing intertwined forming a cylinder, wherein a transverse helicalmagnetic field is formed within the first and second central borecausing a charged particle traveling near the first and secondlongitudinal axis to have acceleration causing the charged particle toradiate.
 3. A permanent magnet twister as in claim 2 wherein:the firstmagnetic orientation and the second magnetic orientation have opposingdirections.
 4. A permanent magnet twister as in claim 2 wherein:saidfirst permanent magnet bar has a rectangular cross section having afirst axial thickness along the first longitudinal axis; and said secondpermanent magnet bar has a rectangular cross section having a secondaxial thickness along the second longitudinal axis.
 5. A permanentmagnet twister as in claim 4 wherein:said first permanent magnet barshaped in the form of a helix has a first plurality of coils, each ofsaid first plurality of coils spaced from each other a distance equal tothe first axial thickness; said second permanent magnet bar shaped inthe form of a helix has a second plurality of coils, each of said secondplurality of coils spaced from each other a distance equal to the secondaxial thickness, said first axial thickness being equal to said secondaxial thickness; and said first magnetic orientation of the firstpermanent magnet bar and said second magnetic orientation of the secondpermanent magnet bar having opposing directions.
 6. A permanent magnettwister as in claim 4 wherein:the cylinder is a right cylinder.
 7. Apermanent magnet twister comprising:a first permanent magnet bar, saidfirst permanent magnet bar shaped in the form of a helix having a firstcentral bore and a first longitudinal axis, said first permanent magnetbar having a first magnetic orientation in a first direction along thefirst longitudinal axis; a second permanent magnet bar, said secondpermanent magnet bar shaped in the form of a helix having a secondcentral bore and a second longitudinal axis, said second permanentmagnet bar having a second magnetic orientation in a second directionalong the second longitudinal axis, said first permanent magnet bar andsaid second permanent magnet bar being intertwined forming a cylinderhaving an exterior surface, the first magnetic orientation and thesecond magnetic orientation have opposing directions; and two bars of ahigh permeability magnetic material placed between a boundary of saidfirst and second permanent magnet bars, each of said two bars of saidhigh permeability magnetic material being shaped in the form of a helix,wherein a transverse helical magnetic field is formed within the firstand second central bore causing a charged particle traveling near thefirst and second longitudinal axis to have acceleration causing thecharged particle to radiate.
 8. A permanent magnet twister as in claim 7wherein:said high permeability magnetic material has a rectangular crosssection and extends a radial distance outward less than that of theexterior surface of the cylinder.
 9. A permanent magnet twister as inclaim 7 wherein:said high permeability magnetic material has atriangular cross section having a vertex.
 10. A permanent magnet twisteras in claim 9, wherein:the vertex of said high permeability magneticmaterial points to the exterior surface of the cylinder.
 11. A permanentmagnet twister as in claim 7 wherein:said high permeability magneticmaterial is iron.
 12. A permanent magnet structure for guiding a chargedparticle, comprising:a helical permanent magnet having a plurality ofcoils and a hollow central bore with a longitudinal axis; each of saidplurality of coils being spaced from each other a first distance equalto the axial thickness of said permanent magnet bar; a charged particlesource, said charged particle source directing a charged particle nearsaid longitudinal axis; said helical permanent magnet having a magneticorientation parallel to the longitudinal axis such that a north pole isformed on a first surface having a plane perpendicular to thelongitudinal axis and a south pole is formed on a second surface havinga plane perpendicular to the longitudinal axis, whereby a spiralingtransverse magnetic field is created between transversely diametricallyopposed north and south poles; and said spiraling transverse magneticfield causing said charged particle travelling near said longitudinalaxis to have acceleration causing said charged particle to spiral.
 13. Apermanent magnet structure as in claim 12 wherein:a second distance in alongitudinal direction between the first surface and the second surfaceis greater than a diameter of the hollow central bore.